VCSEL (vertical cavity surface emitting laser) epitaxial wafer is the core substrate for VCSEL chip manufacturing, and its characteristics directly determine the performance, reliability, and cost of the final laser. For beginners, there is no need to delve into complex epitaxial growth theories. By grasping the following key characteristics, they can quickly establish relationshipsVCSEL epitaxial waferThe core cognition.
1、 Core structural feature: 'Vertical cavity' is fundamental
The core feature of VCSEL epitaxial wafers is the "vertically stacked resonant cavity structure", which is also the most essential difference between it and traditional edge emitting laser epitaxial wafers. This structure is mainly composed of three vertically stacked parts, and the thickness and composition of each layer are precisely controlled:
Distributed Bragg reflector (DBR): It is made by alternately growing multiple layers of semiconductor materials with different refractive indices, equivalent to an "optical reflector", which can confine light in the cavity and reflect it repeatedly, forming laser oscillation. The reflectivity of DBR directly affects the light output efficiency of the laser, and the reflectivity of high-quality epitaxial circular DBR usually needs to reach over 99%.
Active region: located between the upper and lower DBRs, it is the "core region for generating laser light" and usually adopts a multi quantum well (MQW) structure. The number of layers, thickness, and material composition (such as InGaAs/GaAs) of quantum wells determine the emission wavelength (commonly 850nm, 940nm, etc.) and optical power density of VCSEL.
Restriction layer and contact layer: The restriction layer is used to constrain charge carriers and light fields, improving luminescence efficiency; The contact layer provides good conductivity for subsequent electrode preparation, and its doping concentration and uniformity will affect the electrical performance of the chip.
2、 Key performance characteristics: directly related to device performance
The performance characteristics of epitaxial wafers are the "innate foundation" for subsequent chip manufacturing, with a core focus on the following three points:
Wavelength uniformity: The wavelength deviation of VCSEL emission at different positions on the same wafer needs to be extremely small (usually within ± 2nm). If the wavelength uniformity is poor, it will lead to inconsistent performance of mass-produced chips, affecting the application effect of optical communication, sensing and other scenarios. This characteristic is directly determined by the uniformity of temperature and gas flow rate during the epitaxial growth process.
Light output efficiency: mainly depends on the quantum efficiency of the active region and the reflection efficiency of the DBR. Efficient epitaxial wafers can enable VCSEL to output higher optical power at lower driving currents, reducing energy consumption and chip heat generation, thereby improving reliability. The optical output efficiency of chips corresponding to high-quality epitaxial wafers in the industry can reach over 30%.
Crystal quality: With "dislocation density" as the core indicator, dislocations are defects generated during the crystal growth process that capture charge carriers like "impurities", leading to a decrease in optical power and a shortened lifespan. high qualityVCSEL epitaxial waferThe dislocation density of the chip needs to be controlled below 10 ³ cm ² to ensure its long-term reliability (the service life usually requires more than 100000 hours).
3、 Process and application adaptation characteristics: matching production and scene requirements
In addition to the core structure and performance, the process adaptability and scene matching of epitaxial wafers are also crucial, directly affecting production efficiency and application effectiveness
Wafer size and flatness: The current mainstream sizes are 4 inches and 6 inches. Larger sizes (such as 8 inches) can increase the chip output per wafer and reduce unit costs. At the same time, the flatness of the wafer (usually requiring a warpage of ≤ 50 μ m) will affect the accuracy of subsequent photolithography, etching, and other processes, avoiding a decrease in chip yield.
Doping uniformity: The uniformity of doping concentration in epitaxial layers (such as n-type and p-type doping in DBR layers) can affect the threshold current and voltage consistency of the chip. If the doping is uneven, some chips may experience problems such as excessive driving current and severe heating, which increases the screening cost.
Application scenario adaptation: Different scenarios have different requirements for the characteristics of VCSEL epitaxial wafers. For example, consumer electronics (such as facial recognition) require a focus on wavelength stability and low cost when using wafers; Industrial sensing wafers need to enhance their reliability at high temperatures; Optical communication wafers have higher requirements for optical output efficiency and modulation speed.
4、 Beginner Key Reminder
-The characteristics of epitaxial wafers are "predetermined", and subsequent chip processes are difficult to compensate for congenital defects. When selecting, priority should be given to confirming whether the core indicators match the requirements;
-Different epitaxial growth techniques (such as MOCVD, MBE) can affect wafer characteristics, and MOCVD is currently the mainstream technology route due to its good mass production and controllable cost;
-During acceptance, special attention should be paid to key detection reports such as wavelength uniformity and dislocation density to avoid affecting subsequent production due to substandard indicators.